The use of photosensitive etch-resistant materials in microlithography is well known. Generally, as part of the process of fabricating an integrated circuit or other microelectronic devices, a layer of such material, known as a photoresist, is deposited onto a silicon wafer or other material which is to be selectively etched. The photoresist layer is then exposed to an image or pattern of light, which chemically alters the areas exposed to light.
If the photoresist is of the type known as a positive photoresist, then the areas of the photoresist exposed to light can be dissolved or washed away with a developer. With a negative photoresist, the areas not exposed to light are removed by the developer. In either case, the developer is typically a liquid aqueous or organic solution. However, for certain types of photoresists known as dry developed resists, plasma etching may be used to remove the selected parts of the photoresist layer.
After part of the photoresist layer has been removed, the wafer may be immersed in or subjected to an etchant, which removes the exposed areas of the wafer underlying the photoresist. Thus, the light pattern previously projected onto the photoresist is etched into the wafer. The remaining photoresist may then be removed with the use of a suitable solvent. This series of steps is one of the basic processes used in the fabrication of integrated circuits.
A variety of methods can be used to project an image or pattern of light onto the photoresist layer. One such method is to place a mask bearing the desired pattern directly onto the photoresist layer and then expose the mask and wafer to light. In this case, the mask must be of the same size as the image desired on the photoresist layer.
Another method is to project light from a light source through a mask-like object known as a reticle. The reticle typically comprises a transparent layer partially covered by an opaque layer, the opaque layer being a negative image of the circuit or pattern to be etched into the silicon wafer. The light which passes through the reticle is projected through a series of lenses onto the photoresist layer. Using this method, a reticle which bears a pattern several times larger than the pattern to be projected onto the photoresist layer can be used. Typical ratios between reticle size and projected image size in common use are 2, 2.5, 4, and 5.
Since many identical integrated circuits are typically created on a single silicon wafer, a reticle bearing one, two or four identical integrated circuit patterns is typically projected in a series of exposures onto different regions of a single photoresist layer. A system for accomplishing multiple exposures, known as a stepper, has a movable stage for holding and automatically positioning the silicon wafer in a plane perpendicular to the axis along which the image is being projected. The stepper automatically moves the silicon wafer between exposures so that an array of images is successively projected onto the surface of the silicon wafer.
Another type of exposure tool, known as a scanner, uses a reticle and an optical arrangement similar to a stepper. However, while a stepper uses a distributed light source to expose the entire reticle pattern simultaneously, a scanner uses a thin beam or arc of light which is moved ("scanned") across the surface of the reticle. The beam of light is only wide enough to cover a portion of the reticle. Thus, the light beam must be scanned across the reticle in a regular fashion to expose all the surface area of the reticle. The light beam projects the reticle pattern segment-by-segment onto the wafer, so that eventually the entire reticle image is projected onto the wafer.
Two of the critical parameters of a stepper, scanner, or like projection system are its resolution and depth of focus. The resolution of the system determines the minimum size of the lines or structures which may be accurately projected onto the wafer. The depth of focus determines the amount of "flatness" required for the photoresist layer, or the variation in the distance between the image projection optics and the photoresist layer which can be tolerated without losing the focus of the image at the photoresist layer.
Because of the demand for ever-smaller integrated circuits, high resolution projection systems have become increasingly desirable. In addition, a large depth of focus is desired to accommodate the topography of features underneath the photoresist. However, improvements to resolution will usually result in degradation of the depth of focus, and vice versa. Thus, a method and/or device which improves both resolution and depth of focus in a projection system is desirable.
One known method for improving image quality in a stepper system is to employ a special type of reticle known as a phase-shifting reticle or phase-shifting mask. A phase-shifting reticle has selected transparent areas which are different from the other transparent areas of the reticle. Light passing through the selected areas is shifted in phase by a predetermined amount (e.g. 180.degree.) relative to light passing through the other transparent areas of the reticle. This arrangement significantly reduces the "zeroth-order" or diffused background light which reaches the photoresist layer. As a result, the image projected onto the photoresist layer is clearer and sharper, having better resolution and depth of focus than an image projected using a non-phase-shifting reticle. To further improve the resolution and depth of focus of microlithographic imaging systems, a method or system which further reduces the zeroth-order light in the image is desirable.
In addition, a reticle will sometimes contain flaws or imperfections in its pattern which, if left uncorrected, will result in a flawed pattern being etched into the wafer. Therefore, a method or system which detects or corrects flaws in the reticle of a microlithographic imaging system is desirable.